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    SubjectRe: Kernel 4.7rc3 - Performance drop 30-40% for SPECjbb2005 and SPECjvm2008 benchmarks against 4.6 kernel
    On Fri, Jun 24, 2016 at 02:44:07PM +0200, Vincent Guittot wrote:

    > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > > index 22d64b3f5876..d4f6fb2f3057 100644
    > > --- a/kernel/sched/fair.c
    > > +++ b/kernel/sched/fair.c
    > > @@ -2484,7 +2484,7 @@ static inline long calc_tg_weight(struct task_group *tg, struct cfs_rq *cfs_rq)
    > > */
    > > tg_weight = atomic_long_read(&tg->load_avg);
    > > tg_weight -= cfs_rq->tg_load_avg_contrib;
    > > - tg_weight += cfs_rq->load.weight;
    > > + tg_weight += cfs_rq->avg.load_avg;
    >
    > IIUC, you are reverting
    > commit fde7d22e01aa (sched/fair: Fix overly small weight for
    > interactive group entities)

    Ah!, I hadn't yet done a git-blame on this. Right you are, we should
    have put a comment there.

    So the problem here is that since commit:

    2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")

    load.weight and avg.load_avg are in different metrics. Which completely
    wrecked things.

    The obvious alternative is using:

    scale_load_down(cfs_rq->load.weight);

    Let me go run that through the benchmark.

    > I have one question regarding the use of cfs_rq->avg.load_avg
    > cfs_rq->tg_load_avg_contrib is the sampling of cfs_rq->avg.load_avg so
    > I'm curious to understand why you use cfs_rq->avg.load_avg instead of
    > keeping cfs_rq->tg_load_avg_contrib. Do you think that the sampling is
    > not accurate enough to prevent any significant difference between both
    > when we use tg->load_avg ?

    I'm not entirely sure I understand your question; is it to the existence
    of calc_tg_weight()? That is, why use calc_tg_weight() and not use
    tg->load_avg as is?

    It seemed like a simple and cheap way to increase accuracy, nothing more
    behind it until the commit you referred to.

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